Ceramic materials are hard, brittle, heat- and corrosion-resistant substrates made by shaping and then heating a non-metallic mineral, such as clay, at a high temperature. Enamels, porcelain, and bricks are examples of materials that are produced by molding or shaping minerals and baking or firing them at high temperatures.
Glass products are typically made by fusing silicates with boric oxide, aluminum oxide, or phosphorus pentoxide at high temperatures. They have highly variable mechanical and optical properties and solidify from the molten state without crystallization into a transparent or translucent form. While glass items are generally hard and brittle, their lack of crystalline structure puts them in the class of amorphous solids. Glass items that may require printed graphics include windows, mirrors, cooling utensils, bottles, containers, and more.
From a printing point of view, several methods are available for decorating glass and ceramics with high-quality images. The printing processes used for glass and ceramic printing rely on a variety of ink systems. Other than sublimation inks, most inks fall into one of two families: organic and inorganic.
Organic inks are typically used in screen, inkjet and pad printing, and consist of organic pigments and resins along with other chemistries that cure over time and rely on temperature or some other form of energy to create a bond with the substrate. The most effective organic inks are produced as two-component or two-part systems. These inks generally contain resins capable of polymerization that are blended with catalysts to initiate polymerization. Heating the products to a temperature of approximately 200° C. after printing may accelerate the curing process and improve adhesion. In addition, such heat exposure will typically enhance the mechanical and chemical resistance of the print. After printing, organic ink films will require at least 48 hr to polymerize unless heat is applied.
Inorganic inks use mineral-based pigments and materials that, once printed, have to be heated and melted at high temperatures in order to combine with the substrate surface and form a permanent bond.
Ceramic colors, as inorganic ceramic inks are called, are a mixture of pigments (metal oxides and salts) and finely ground glass particles, called frit. These materials are fused to the substrate by calcining (“firing”) them at temperatures between 600-1450° C. Firing temperatures vary depending on the make-up of the color, the nature of substrate, and other application criteria, but in all cases the temperatures must be carefully controlled to achieve specific colors after firing.
These high firing temperatures are used as the components of ceramic colors need to be melted so they can fuse to the ceramic surface on which they are printed. While these inks are typically called “inorganic”, they may also contain small amounts of organic material. The organic components are the materials in which the pigment and frit are suspended to create a printing ink. These organic materials, which are oily in nature, are designed to burn off rapidly during firing without affecting print quality and final color.
Inorganic inks come in various forms. These include screen- and pad-printable process-color formulations, thermoplastic varieties, and total-transfer inks. Both the screen-printing and the total-transfer systems are known as “cold color” inks, which mean they do not have to be heated to become printable while the thermoplastic inks must be heated before they can be applied to the substrate.
Thermoplastic ink systems are waxy at room temperature and have to be heated up for printing. For pad printing, the ink trough, plate, and occasionally the pad are kept at a temperature of approximately (60° C.). When the pad carrying the ink comes into contact with the cold object to be printed, the ink cools and sticks to the object.
When screen printing with thermoplastic inks, the mesh is made from stainless steel and an electric current is passed through it. This heats up the screen and melts the ink, which then flows through the mesh and solidifies when it makes contact with the cold ceramic or glass. Controlling current flow is critical because too much will overheat the color and burn out the mesh.
While printing on ceramic surfaces with organic inks may be obtained by silk-screen, pad printing or digital printing, commercial ceramic inks are difficult to use in inkjet printing as they have typically a viscosity higher than required for inkjet printing (about 20-40 cps) and the glass frit contained in them, which is in the micron size range, tends to sediment and also clog the nozzles on the orifice plate from which the ink is jetted during inkjet printing.
It would have been highly desirable to use inkjet printing on the ceramic surfaces with ceramic pigments, instead of the currently used methods of silk-screen, or pad printing. Converting to digital printing can have the following advantages: reduction of costs involved with storage of screens or transfer devices due to digital storing of the desired patterns instead of physical storage; reduction of costs for low value printing which can be prohibitive in silk-screen printing; increase the ease and versatility of switching from one design to another, capacity for edge to edge printing utilization.
Attempts at printing ceramics colors by the inkjet process and hence making the inkjet process available also for decorating ceramic articles, such as glass, enamel and porcelain, have hitherto always failed owing to the pronounced tendency of the specifically heavy and coarse color powders to form sediment. The specific gravity of overglaze and glass colors is from 3.5 to 6.0 kg/l, and the mean fineness of grain of those products is from 3 to 5 μm. Those products settle out from aqueous or alcoholic suspensions having the conventional inkjet processing viscosity within a few seconds to the extent of in some cases 50%. Such suspensions would rapidly lead to blocking of the printing nozzles and of the entire inkjet printing apparatus. Liquid color pastes having a substantially higher viscosity, for example 5000 mPa.s, such as are used in screen printing for decorating glass, will not be suitable for printing by ink jet, since the viscosity is much too high for the ink jet printers present nowadays.
U.S. Pat. No. 6,357,868, incorporated herein by reference, discloses a method for decorating ceramics by inkjet technology using inorganic pigments and glass frit present in a thermoplastic medium having a melting point of at least 30° C. (such as wax). The medium, which is solid under storage conditions, eliminates settling out of the inorganic pigments. The ink is melted just before printing by use of a heatable inkjet print head.
According to this patent sedimentation is eliminated by the use of a thermoplastic medium that solidifies immediately.
EP 1,223,201, incorporated herein by reference, discloses an ink for printing on heat resistant substrates comprising pigment, fusible vitreous agents having particles of less than 10μ and a carrier. The carrier according to this patent is also thermoplastic having a high melting point for phase change of the ink. Such an ink, which is solid under room temperature, needs to be heated prior to printing.
Ink which is solid at room temperatures and has to be heated at the inkjet head prior to printing is awkward to handle, load, and requires special equipment for actual printing. If the ink is liquid at room temperature, it will make the performance and maintenance of the printer much better.